This invention generally relates to a steering fluid device and method for use in an asymmetric heat-type inkjet printer that increases the angle of deflection of the ink droplets generated by the nozzles in the printhead.
Many different types of digitally controlled printing systems have been invented, and many types are currently in production. These printing systems use a variety of actuation mechanisms, a variety of marking materials, and a variety of recording media. Examples of digital printing systems in current use include: laser electrophotographic printers; LED electrophotographic printers; dot matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers; and inkjet printers. However, at present, such electronic printing systems have not significantly replaced mechanical presses, even though this conventional method requires very expensive set up and is seldom commercially viable unless a few thousand copies of a particular page are to be printed. Thus, there is a need for improved digitally controlled printing systems that are able to produce high quality color images at a high speed and low cost using standard paper.
Inkjet printing is a prominent contender in the digitally controlled electronic printing arena because, e.g., of its non-impact low-noise characteristics, its use of plain paper, and its avoidance of toner transfers and fixing. Inkjet printing mechanisms can be categorized as either continuous inkjet or drop on demand inkjet. Continuous inkjet printing dates back to a least 1929. See U.S. Pat. No. 1,941,001 to Hansell.
Conventional continuous inkjets utilize electrostatic charging tunnels that are placed close to the point where the drops are formed in a stream. In this manner individual drops may be charged. The charged drops may be deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter (sometimes referred to as a Acatcher@) may be used to intercept the charged drops, while the uncharged drops are free to strike the recording medium.
A novel continuous inkjet printer is described and claimed in U.S. patent application Ser. No. 08/954,317 filed Oct. 17, 1997, and assigned to the Eastman Kodak Company. Such printers use asymmetric heating in lieu of electrostatic charging tunnels to deflect ink droplets toward desired locations on the recording medium. In this new device, a droplet generator formed from a heater having a selectively-actuated section associated with only a portion of the nozzle bore perimeter is provided for each of the ink nozzle bores. Periodic actuation of the heater element via a train of uniform electrical power pulses creates an asymmetric application of heat pulses to the stream of droplets to control the direction of the stream between a print direction and a non-print direction.
While such continuous inkjet printers have demonstrated many proven advantages over conventional inkjet printers utilizing electrostatic charging tunnels, the inventors have noted certain areas in which such printers may be improved. In particular, for reasons not entirely understood, the inventors have noted that some ink droplets may become misdirected during the printing operation, and either strike the printing medium when they should have been captured by the gutter, or vice versa. While the incidence of such misdirected droplets is small, any such misdirection frustrates the goal of 100% accuracy in the printing operation. The inventors have also observed that a possible solution to the problem of droplet misdirection might be the replacement of water-based inks with inks based upon organic solvents such as isopropanol. Such organic solvents have a higher volatility and lower heat capacity than water. Hence, a stream of ink based on such solvents will deflect more sharply in response to heat pulses generated by the heater placed adjacent to the nozzle outlet. Unfortunately, the use of inks based on such organic solvents generates environmental problems since such solvents are more hostile to the environment and more expensive to dispose of than water-based inks.
Clearly, there is a need for an improved, asymmetric heat-type inkjet printer, which is capable of increasing the angle of deflection of the ink droplets without the use of environmentally objectionable ink chemistries. Ideally, such an improvement would be simple and inexpensive to implement in existing print heat designs.
Generally speaking, the invention is an ink drop generator for printhead that overcomes or ameliorates all of the aforementioned disadvantages associated with the prior art. To this end, the invention comprises an inkjet printhead having at least one nozzle for continuously ejecting a stream of ink that forms a train of ink droplets; a heater disposed adjacent to the nozzle for selectively thermally deflecting the droplet-forming stream of ink, and a steering fluid assembly for providing a film of fluid around the droplet-forming stream that is more deflective in response to heat pulses generated by the heater than the ink.
The steering fluid assembly may include a pair of bores in the inkjet printhead which communicate with opposing sides of the side walls of the nozzle for uniformly injecting a film of steering fluid around the droplet-forming ink stream such that a co-extruded jet is formed comprising a cylindrical core of ink surrounded by an annular film of steering fluid. In the preferred embodiment of the droplet generator, the ink is an aqueous-based mixture, and the steering fluid is a liquid having a higher volatility and lower thermal diffusivity than the ink. The steering fluid may be one of the group consisting of alcohols, glycols, surfactants, and micro-emulsions. Specific compounds suitable for use as steering fluids include polypropylene oxide, polyethylene oxide, and isopropanol.
The fluid-conducting bores of the steering fluid assembly are each connected to a pressurized supply of steering fluid so that a co-extruded stream of steering fluid and ink is produced. In one preferred method of the invention, the flow rate of the steering fluid is adjusted relative to that of the stream of ink ejected from the outlet of the nozzle so that an annular film of steering fluid between 0.1 and 1.0 microns in depth surrounds a cylindrical stream of ink approximately 8 microns in diameter. In another preferred method, only one of the bores of the steering fluid assembly is used to introduce steering fluid into the stream, which results in an asymmetric co-extended stream of ink and steering fluid. In this mode of operation, the bore that introduces the steering fluid is preferably placed on the same side of the nozzle as the heater to ensure that the resulting, co-extruded stream includes a film of steering fluid on the side of the stream nearest the heater. In a third preferred method, steering fluid is introduced through only one bore of the steering fluid assembly whenever deflection is needed. Hence, droplet deflection occurs as a result of the modulation of the flow of steering fluid through a single bore. In this method, the location of the bore need not depend on the location of the heater, as the heater is not used to deflect the stream.
By increasing the angle of deflection of the ink stream by the heater, the inkjet printhead of the invention may be more closely positioned to the printing medium, thereby increasing the accuracy (and hence clarity) and speed of the printing operation. The use of only a thin film of steering fluid minimizes any adverse environmental effects associated with the use of volatile organic liquids.